1. Field of the Invention
This invention relates to light emitting apparatus and light emitting method that can be applied to a field requiring reliability such as disaster prevention light and traffic light, a field requiring light weight and small size such as in-vehicle light and LCD backlight and a field requiring visibility such as station guide plate, and more particularly to light emitting apparatus and light emitting method that has a structure with light emitting element and phosphor combined.
2. Description of the Related Art
Conventionally, a light emitting apparatus is developed that part of light from light emitting element is wavelength-converted, then mixed with light not wavelength-converted to offer light with emission color different from that of the light emitting element.
For example, a light emitting apparatus is commercially available that employs a III group nitride system compound semiconductor light emitting element to emit blue light, and a cerium (Ce) doped yttrium-aluminum-garnet system phosphor (YAG). In this light emitting apparatus, the light emitting element is mounted on the cup portion of lead frame and light transmitting material with phosphor (YAG) dispersed is filled in the cup portion, thereby phosphor layer is formed in the emission direction of the light emitting element. In operation, part of light from the light emitting element is absorbed and wavelength-converted by the phosphor (YAG) when it passes through the phosphor layer, the other part of light is transmitted through the transmitting material without being absorbed. Accordingly, these two lights are mixed and, therefore, white light can be radiated from the light emitting apparatus.
Well known phosphors are composed of matrix material, such as silicates, phosphates (e.g., apatite) and aluminates, and transition metals or rare-earth metals to be doped into the matrix material.
On the other, few phosphors are known that use matrix material of nitride or oxide nitride with transition metals or rare-earth metals doped thereto.
Known nitride phosphor are manganese activated aluminum nitride (prior art 1, described later), and rare-earth element activated magnesium silicon nitride (MgSiN2) (prior art 5, described later). Recently reported are red phosphor of Mg doped ZnSiN2 with strained wurtzite type structure (prior art 6, described later), red phosphor of Eu doped CaSiN2 (prior art 7, described later) and phosphor of Eu doped Ba2Si5Na.
Known oxide nitride phosphor are phosphor with β-sialon, matrix material (prior art 2, described later), Ce doped oxide nitride (Y—Al—O—N) with silicate mineral or apatite structure (prior art 8, described later), Ba1−xEuxAl11O16N with β-alumina structure (prior arts 9 and 10, described later). Also, recently reported is phosphor with oxynitride glass, matrix material (prior arts 3 and 4, described later).
By the way, white LED (light emitting diode) is used in a field requiring reliability such as disaster prevention light and traffic light, a field requiring light weight and small size such as in-vehicle light and LCD backlight and a field requiring visibility such as station guide plate. The emission color of white LED, i.e., white, is obtained by mixing lights with different emission colors. Namely, blue light generated from InGaN system blue LED with emission wavelength of 450 to 550 nm is mixed with yellow light generated from phosphor.
Such a white LED frequently uses, as phosphor, Ce doped YAG system oxide represented by: (Y, Gd)3(Al, Ga)5O12. This phosphor is coated on the surface of blue LED and radiates white light based on the principle described above.                Prior art 1: German patent No. 789,890        Prior art 2: Japanese patent application laid-open No. 60-206889        Prior art 3: Japanese patent application laid-open No. 2001-214162        Prior art 4: Japanese patent application laid-open No. 2002-76434        Prior art 5: “Izv. Akad. Nauk SSSR, Neorg. Master” 17(8), 1431-5        Prior art 6: T. Endo et al., “High pressure synthesis of periodic compound and its optical and electrical properties”, In T. T suruta, M. Doyama and Seno (Editors), New Functionality Materials, Volume C, Elsevier, Amsterdam, The Netherlands, pp. 107-112 (1993)        Prior art 7: S. S. Lee et al., “Photoluminescence and Electroluminescence Characteristic of CaSiN2:Eu”, Proc. SPIE-Int, Soc. Opt. Eng., 3241, 75-83 (1997)        Prior art 8: J. W. H. van Krevel et al., “Long wavelength Ce3+ emission in Y—Si—O—N materials”, J. Alloys and Compounds, 268, 272-277 (1998)        Prior art 9: H. Hintzen et al., “On the Existence of Europium Aluminum Oxynitrides with a Magnetopolumlite or β-Alumina-Type Structure”, J. Solid State Chem., 142, 48-50 (1999)        Prior art 10: S. R. Jansen et al., “Eu-Doped Barium Aluminum Oxynitride with β-Alumina-Type Structure as New Blue-Emitting Phosphor”, J. Electrocher. Soc., 146, 800-806 (1999)        
However, in the conventional light emitting apparatus, which is composed of blue LED and YAG:Ce, its emission light have weak red component. Therefore, it lacks in color rendering property as to red color.
Also, oxide system phosphors generally have a problem that the spectrum intensity lowers significantly when the excitation wavelength exceeds 400 nm. Therefore, in white LED that YAG system oxide phosphor is coated on the surface of InGaN system blue LED, the excitation energy of YAG system oxide phosphor does not coincide with the excitation energy of blue LED light source, and therefore the excitation energy cannot be efficiently converted. Thus, it is difficult to enhance the brightness of white LED.